radeonsi: try flushing unflushed fences in si_fence_finish even when timeout == 0

radeonsi: try flushing unflushed fences in si_fence_finish even when timeout == 0

Under certain conditions, waiting on a GL sync objects should act like
a flush, regardless of the timeout.

Portal 2, CS:GO, and presumably other Source engine games rely on this
behavior and hang during loading without this fix.

Fixes: bc65dcab3bc4 ("radeonsi: avoid syncing the driver thread in si_fence_finish")
Signed-off-by: Marek Olšák <marek.olsak@amd.com>
Tested-by: Kai Wasserbäch <kai@dev.carbon-project.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=103902
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=103904